There are known in the prior art plain paper electrostatic copying machines in which a drum carries a surface coating of photoconductive material such, for example, as selenium. The surface of the drum is moved successively past a charging station at which a corona applies uniform electrostatic charge over the surface of the drum and then to an exposure station at which the charged surface is exposed to an image of the original to be copied. In one particular form of such machine, a scanning system reciprocates below a glass platen on which the original is placed face down so that a line image of the original is translated to the moving drum surface at the exposure station. Further, as is known in the art, the surface of the drum in relatively light areas loses its charge while retaining its charge in relatively darker areas representing the printed areas of the original, so that the drum surface upon leaving the exposure station carries a latent electrostatic image of the original to be copied.
After having left the exposure station, the drum surface moves through a developer station at which a developer carrying particles of toner is applied to the drum surface so as to develop the latent image. Certain machines employ dry developer systems while others employ wet developer systems.
Upon leaving the developer station, the surface of the photoconductive drum enters a transfer station at which the developed image on the drum is transferred to the surface of a sheet of plain paper fed to the drum so as to pass through the transfer station in contact with the drum. Transfer may be effected, for example, by means of a transfer corona.
Following the transfer operation, the copy sheet to which the image has been transferred is picked-off the surface of the drum and is moved into a delivery system which transports the copy out of the machine to a tray, or the like, at which it is accessible to the user. In one particular type of pick-off system employed in the prior art, a narrow strip of the copy material extending rearwardly from the leading edge along a side thereof moves over a stationary flexible band conforming to the configuration of the drum over a portion of the circumference thereof and mounted in closely spaced relationship thereto at the edge thereof. At the point at which the picking-off operation is to take place, a struck out portion of the band moves the corner of the sheet forming the initial part of the narrow strip referred to hereinabove away from the surface of the drum and into the nip between a take-off roller and a belt. This operation is sufficient to reverse the direction of movement of the sheet and to feed it into the nip between transport rollers which move the sheet into a drying system from which the sheet passes to delivery rollers which carry the sheet out of the machine to the delivery tray, or the like.
A machine of the type described above is provided with a cleaning system, located beyond the pick-off station with reference to the direction of drum movement, which operates to remove any residual toner particles from the surface of the drum before the next operation of the machine takes place. More particularly, in one particular type of cleaning system, a sponge roller in engagement with the surface of the drum rotates in such a direction that the portion of its surface which is in engagement with the drum surface moves in the opposite direction thereto. This sponge roller normally is wet with developer to facilitate the cleaning action. It will readily be appreciated that, after a period of time in use, this roller becomes relatively black in appearance.
A further feature of the machine of the type described hereinabove is an arrangement for indicating a misfeed to the machine operator to permit the operator to correct the condition before the next machine operation. When such a condition occurs, a portion of the machine cabinet can be opened to permit the user to view a limited portion of the machine, including the reversing roller which cooperates with the belt to form the nip into which the picked-off sheet is fed as well as the forwarding rollers and hold-down roller of the dryer system. If the user observes the misfed paper at the reversing roller, for example, or at the hold-down roller, these rollers can manually be removed to permit the misfed paper to be taken out of the machine. Before the machine can again be set in operation, however, a reset push-button must be operated to restore the electrical circuitry to a condition at which the machine can be operated.
While the machine described hereinabove functions satisfactorily in most instances it incorporates a serious disadvantage. That is, a sheet of copy paper carrying a developed image need be only slightly skewed or misaligned, or otherwise not properly oriented, when fed from the paper supply cassette to the drum in order completely to miss the metal strip comprising the pick-off element of the system.
Machines of the type described customarily are provided with cassettes containing supplies of paper of standard widths which are fed to the machine to receive the image from the drum. In some instances, it may be desirable to make a copy on a sheet which is narrower than the standard width. While this can be successfully achieved so long as the edge of the paper to be picked off the drum is aligned with the pick-off device, an inexperienced or careless operator may cause the narrow sheet to be fed to the drum with the edge to be engaged by the pick-off device out of alignment therewith.
In either of the two cases described hereinabove, the sheet will continue its travel and is likely to become wrapped around the cleaner roll. Under such conditions, the machine will indicate that a misfeed has occurred. In response to the indication, the operator will open the access door at the delivery end of the machine so as to be able to view the return roll and the other rollers of the delivery system. The cleaning roller, however, is not readily visible with the access door open and, in any event, a sheet of paper which has become wrapped around the roller rapidly is saturated with developer containing carbon black toner particles so that, even if the roller is partially visible, the operator will not readily observe the paper which has become wrapped around the roller. It is most likely that the usual operator will, as has in many instances actually occurred, assume that no misfeed exists. The operator then pushes the reset button and the machine is ready for the next operation.
The selenium surface of the photoconductive drum is notoriously prone to damage such as scratching, or the like, by any instrument or rough surface with which it may come into contact. The misfed sheet described hereinabove which becomes wrapped around the cleaner roll is wrinkled and dries out when the machine is not in use so as to form portions which will readily scratch the selenium surface when the machine is again set into operation. Under these conditions, the effectiveness of the surface in making copies is destroyed in a relatively short period of time. As a result, it becomes necessary to replace the drum which is one of the most expensive components of the machine.
We have invented a misfeed detector which overcomes the defects of machines of the type described hereinabove. Our misfeed detector prevents a copy which has missed the pick-off element from reaching the cleaning roll of the machine. One form of our jam detector disables the machine in response to a copy which has missed the pick-off element and prevents resetting of the machine until this copy is physically removed from the machine. Another form of our misfeed detector returns to the operator a copy sheet which has missed the pick-off element. Our device is readily installed on existing machines of the type described. It is relatively inexpensive for the result achieved thereby. It is certain in operation.